Changing winds Intense tropical cyclones are moving further away from the equator, an analysis of historical storm data reveals.

Over the past 30 years, the location where these powerful storms reach their maximum intensity has shifted toward the poles in both the northern and southern hemispheres at a rate of about 56 kilometres per decade — or half a degree of latitude per decade — US researchers report today in Nature.

The scientists tracked the trend using satellite and other data spanning from 1982 to 2012 collected by NOAA's National Climatic Data Center.

"Historical intensity estimates can be very inconsistent over time, but the location where a tropical cyclone reaches its maximum intensity is a more reliable value and less likely to be influenced by data discrepancies or uncertainties," says Kossin.

While the study documented the trend over a period of three decades using the data, it may very well have begun well before, he says.

The study shows the amount of poleward migration varies by region, with the greatest migration in tropical cyclones in the northern and southern Pacific and south Indian Oceans.

However, this march away from the equator was not seen in the Atlantic, although hurricanes have registered increases in average intensity due to factors that may be counteracting the poleward trend seen elsewhere, say the researchers.

Shifting risk zones

The movement of maximum intensity means regions further north and south of the equator that have not dealt with many of these storms may now be in the crosshairs.

While regions closer to the equator may experience a reduced risk of being battered from intense storms, areas that rely on rainfall spawned by these storms may experience water shortages as the storms migrate away from them, say the researchers.

Gabriel Vecchi, an atmospheric scientist at NOAA's Geophysical Fluid Dynamics Laboratory, says the researchers could not say with certainty whether increases in greenhouse gases or stratospheric ozone depletion have caused the poleward movement.

But Vecchi says the latitude of maximum intensity has moved toward the poles at roughly the same rate as an expansion of the Earth's tropics over the same period, and other studies have attributed the tropics' expansion to human activities.

"Now that we see this clear trend, it is crucial we understand what has caused it — so we can understand what is likely to occur in the years to come," says Vecchi.

Australian region

Unpublished data from the Australian region suggests that the poleward shift is not as strong here as elsewhere in the Pacific around countries such as Fiji and New Zealand, but it could still have an impact, says Dr Hamish Ramsay of Monash University, who wrote an accompanying commentary in Nature.

"If this shift in poleward activity continues then cities and towns further south along the Australian coastlines may be susceptible to more destructive storms," says Ramsay.

"While it would be exaggeration to say Brisbane is under threat, there's certainly a southward jog in the maximum intensity of storms."

But while Kossin and colleagues' findings provide insight into the response of a global tropical cyclone activity to a changing climate, Ramsay says several questions about wind patterns and direction remain unanswered.

"For example, in the Queensland region wind patterns, on average, tend to steer tropical cyclones away from the coast so it's important for us to understand how these wind patterns will change in the future," he says.

"The results from this study just show there's been this poleward shift in the maximal intensity of the storms, but not which way they'll move — whether they'll move to the east or the west — and that has implications for following storms that hit coastlines."